1. Transboundary influences on US background ozone
Huiqun Wang1
Philippe Le Sager2
Rokjin Park3
Daniel Jacob2
Smithsonian Astrophysical Observatory
Harvard University
Seoul National University
Funding from DOE

3. Tightening the standard has large impact

4. Background ozone is important for setting the standard
Fiore et al. [JGR 2003b]
March-October 2001
CASTNet sites observations
GEOS-Chem at CASTNet
NA background
from GEOS-Chem
Background is not directly measurable, and need to be estimated from models

5. NA background – No anthropogenic emissions in North America
US background – No anthropogenic emissions in the US
Anthropogenic
NOx
World
North
America
USA
Canada
Mexico
(Tg N/Year)
25.387
8.052
6.663
0.483
0.905
The national totals of CAN & MEX are small, but
there are non-negligible emission sources near the boarder.

6. NA background (without NA anthropogenic emissions)
JJA mean based on daily-8h-max
US background (without US anthropogenic emissions)

7. Canadian and Mexican Influences (ppb)
JJA mean based on daily-8h-max
sites
Enhancement to NA background is concentrated in the northeast and southwest US

8. JJA mean daily-8h-max Canadian enhancement in the northeast (ppb)
Sites
The largest JJA mean Canadian enhancement is 8.8 ppb (among all US gridboxes).

9. JJA mean daily-8h-max Mexican enhancement (ppb)
Sites
The max JJA mean Mexican enhancement is 13.1 ppb among all US gridboxes.

10. Modeled v.s. observed surface ozone at selected sites
Northeast
r=0.76
Southwest
r=0.60

11. Probability distribution of daily8hmax surface ozone in the U. S
Probability distribution of daily8hmax surface ozone in the U.S. (JJA, 2001)
NA-background
(PRB)
Standard-run
US-background 20 40 60
O3 (ppb)
US background is several ppb higher than NA background

12. Time series at Unionville, MI r = 0.85
Observation Std-Run US-background NA-background CANMEX
Episodic enhancements to NA background increase US background to >40 ppb

13. CAN contributes 59% to O3 precursors on peak ozone days at Buffalo
Slide from EPA presentation

14. At different locations,
transboundary pollutions contribute differently to surface ozone.
CAN influence > 10 ppb on some peak ozone days (>80ppb)
CAN influence > 10ppb does not occur when ozone > 80ppb

15. Time series at Alpine, CA r = 0.64
Observation Std-Run US-background NA-background CANMEX
Larger mean enhancement, but less episodic.
US background is frequently above 30 ppb

16. Canadian pollutions influence the northwest through frontal systems
Some fronts reach far into the U.S.

17. US background and NA background statistics
for successive 10ppb ozone bins
Max
90%
75%
50%
25%
Min
Statistics are for all US gridboxes with JJA mean enhancement >= 3ppb
Southwest has higher background ozone
Max background occur when ozone is 50 – 70 ppb

18. Canadian influence is >10ppb on some peak ozone days (>80ppb)
Canadian influence is always <10ppb on peak ozone days(>80ppb)
Untagged color boxes have max surface ozone < = 80 ppb
Color background : JJA mean Canadian influence as before
Canadian influence is important for the areas near the lakes and northeast coast
Max Canadian contribution on peak ozone days is 23 ppb among the boxes indicated by

19. Canadian influence is >10ppb on some peak ozone days (>75ppb)
Canadian influence is always <10ppb on peak ozone days(>75ppb)
Untagged color boxes have max surface ozone < = 75 ppb
Color background : JJA mean Canadian influence as before

20. Canadian influence is >10ppb on some peak ozone days (>70ppb)
Canadian influence is always <10ppb on peak ozone days(>70ppb)
Untagged color boxes have max surface ozone < = 70 ppb
Color background : JJA mean Canadian influence as before

21. Contribution of CAN pollutions to non-attainment for different choices of ozone standard93 50
28 cases Y > 10ppb
& X > 80ppb
cases Y > 5ppb
& X > 80ppb Y:
138
527
280
854 cases X> 80ppb X:
1679
2936
15180 data points: all JJA timeserises at US gridboxes within [36N-48N, 88W-66W].

22. Color background : JJA mean Mexican influence as before
Mexican influence is >10ppb on some peak ozone days (>80ppb)
Mexican influence is always <10ppb on peak ozone days(>80ppb)
Untagged color boxes have max surface ozone <= 80ppb
Mexican influence is important for southern California
Max Mexican contribution on peak ozone days is 18 ppb among the boxes indicated by

23. Color background : JJA mean Mexican influence as before
Mexican influence is >10ppb on some peak ozone days (>75ppb)
Mexican influence is always <10ppb on peak ozone days(>75ppb)
Untagged color boxes have max surface ozone <= 75ppb

24. Color background : JJA mean Mexican influence as before
Mexican influence is >10ppb on some peak ozone days (>70ppb)
Mexican influence is always <10ppb on peak ozone days(>70ppb)
Untagged color boxes have max surface ozone <= 70ppb

25. Contribution of MEX pollutions to non-attainment for different choices of ozone standard53 28
14 cases Y > 10ppb
& X > 80ppb Y:
49 cases Y > 5 ppb
& X > 80ppb
271
109
413 cases X > 80ppb X:
1002
2453
34960 data points : all JJA timeserises at US gridboxes within [26N-42N,120W-90W]

26. US background = NA background + CANMEX Enhancement50 40 30
US bkg
CANMEX enhancement
All US JJA time series included
NA background
CANMEX enhancement maximizes when NA background is 10 – 20 ppb

27. US background = NA background + CANMEX Enhancement50 40 30 20
For peak ozone cases (>80 ppb) in the US,
Transbounrary pollutions are large (>10ppb) when NA background is 10 – 30 ppb.
They can increase US background to levels >30 ppb.

28. Conclusion
Transboundary pollutions from Canada and Mexico increase the background surface ozone in the US by 1-13 ppb.
The max JJA Mean Canadian enhancement is 8.8ppb, with episodic maximum of 34 ppb (at Unionville) in daily-8h-max time series.
The max JJA mean Mexican enhancement is larger (13.1ppb), but the maximum in daily-8h-max time series is smaller (21ppb, not shown).
The median US background in the southwest is 30-40ppb, and that in the northeast is 20-25ppb.
The maximum US background in the southwest is near 55ppb, and that in the northeast is near 50ppb. They occur when the surface ozone is 50-70ppb.
Canadian and Mexican enhancements maximize when NA background is 10–30ppb.
For peak ozone cases, the US background can be > 30 and even 40 ppb due to transboundary influences.
Transboundary pollutions can contribute significantly (up to 23 ppb in the northeast and 18 ppb in the southwest) to peak ozone levels (>80ppb) sometimes.